1. Field of the Invention
This invention relates generally to printed wiring boards that are made in part using the subtractive etch method. More specifically, the invention relates to a method of producing fine-line circuitry and the resulting product wherein a noble metal is used as an etch mask for the subtractive etch process and the noble metal is also a seed layer for surface finishing via additive plating.
2. Description of Related Art
Certain conventional processes of forming printed wiring board (PWB) circuitry employ a metallic tin-lead etch mask when the subtractive etch method is used. A conventional circuitization process which employs either the electrolytic plating or the electroless plating process for circuitization is as follows: A dielectric substrate is covered with a metal foil base for plating initiation. Next, holes are drilled and they are metallized by thinly seeding them, along with the copper foil, with a palladium or other activator that acts as a catalytic surface for electroless copper deposition. The substrate is plated with a thin layer of electroless copper that electrically connects the holes with the base copper foil. Next, a photoresist is deposited on top of the copper clad laminate and is exposed with the desired pattern and developed. The clad laminate with photoresist then undergoes electrolytic or electroless plating of conductive metal onto the exposed metallized surface of the laminate and onto the inside surface of the holes. A metallic tin-lead etch mask is deposited on the circuitry. Thereafter, the photoresist is stripped off, exposing the underlying metallic foil base in the areas surrounding the circuitry. The metal foil base is then etched away where the photoresist was stripped off leaving, for example, copper/tin-lead circuitry on the surface of the board and on the inner surface of the holes at the completion of the circuitization process.
Although the tin-lead layer can be left on the circuitry as a protective layer, the tin-lead mask often must be removed whenever wiring board circuitry requires subsequent surface finishing. An example of surface finishing can include a nickel layer on top of the copper to prevent corrosion and to act as a diffusion barrier between copper and an additional deposited metal. The nickel is also used to promote adhesion of a gold layer when, for example, an application calls for wire bonding between the circuitry and wire bond silicon die.
The tin-lead strip processes currently employed can be too aggressive or incomplete or both. Aggressive tin-lead etching damages the copper circuitry whereas incomplete etching leaves a tin-lead residue which hinders plating of additional metals on the circuitry. As a result, printed wiring boards requiring surface finishing must be constructed using higher cost and lower yield methods such as full build additive plating or pattern electroplating.
Accordingly, the need exists for a method of making printed circuit boards which eliminates the problems associated with removal of the etch mask.